(a) Areas in an Air Receiver Prone to Corrosion and Reasons for Corrosion Occurrence

Most probable locations of corrosion in an air receiver:

• Bottom of the reservoir: Moisture settles at the lowest point, promoting corrosion.
• Around valve openings: Frequent condensation and temperature changes promote localized corrosion.
• Drain opening and surroundings: Presence of condensate and oil residues contributes to corrosion.
• Near the fusible plug: Exposure to heat and moisture makes this area vulnerable.
• Weld beads and manhole areas: Inconsistent surface finish and potential for residual stress contribute to corrosion susceptibility.
• Inner welds of the compensating ring: These are more exposed to corrosive elements due to structural geometry.
• Chain pitting and line corrosion: May occur along the full length and circumference of vertically mounted receivers. These consist of narrow pits or corroded cavities, typically of limited width but significant depth.

(b) Causes of Corrosion in Air Receivers

1. Oxidation Corrosion: Occurs due to the reaction of steel with oxygen, moisture, and oil in high-pressure air.
2. Weak Acid Corrosion: Moisture and oil vapors condense in cooler areas forming weak acids, which attack the metal surfaces.
3. Galvanic Corrosion: Uneven distribution of condensate leads to micro galvanic cells; water droplets create anodic zones resulting in pitting corrosion.
4. Stress Corrosion: The air bottle is under constant tensile stress; in the presence of a corrosive environment, this can lead to cracking and structural weakening.
5. Fatigue Corrosion: Pressure fluctuations, especially during maneuvering, cause alternating stresses that promote fatigue failure in corroded areas.

(c) Measures to Minimize Corrosion in Air Receivers

1. Operational Practices:
   • Regular draining of the air bottle by watchkeepers to prevent moisture accumulation.
   • Avoid excessive cut-in and cut-off cycles; use deck or service air compressors for auxiliary purposes.
2. Maintenance of Compressor and Ancillary Systems:
   • Maintain compressors in optimal condition.
   • Clean or replace filters, air coolers, and spring-loaded valves regularly to ensure high-quality air supply.
3. Inspection and Preventive Maintenance:
   • Internal Inspections: Conducted semi-annually. Includes thorough cleaning, rust control, and application of protective coatings.
   • After disconnecting all fittings, the interior should be cleaned and inspected for corrosion, especially at weld seams.
   • Post-inspection, ensure the interior is free from scale and foreign matter.
4. External Inspections:
   • Clean the receiver surface using warm water.
   • Visually check for signs of corrosion, scoring, distortion, and damage, with special attention to weld seams.
   • If storage is needed after cleaning, seal all openings to prevent dust and moisture ingress.

Calculation of Maximum Permissible Working Pressure After Corrosion

Given Formula:

$$\sigma=\frac{P.\:d}{2t}$$

where:

σ = hoop stress

P = working pressure (N/m²)

d = diameter (m)

t = wall thickness (m)

Rearranged formula to find new allowable pressure (P₂):

$$P_2=P_1.\frac{t_2}{t_1}$$

Where:

P1​ = original design pressure

t1​ = original wall thickness

t2​ = measured, reduced thickness due to corrosion

Example:

For an original wall thickness of 18 mm with a 1.5 mm corrosion allowance, if the measured thickness is less, calculate P2​ accordingly using the above formula.

(d) Actions Chief Engineer Must Take Upon Discovering a Corroded Receiver

1. Operational Adjustments:
   • Designate the affected air receiver as a standby unit.
   • Monitor draining closely to minimise retained moisture.
2. Pressure Adjustments:
   • Calculate the new allowable working pressure P2​.
   • Reset the cut-in and cut-off pressure limits based on the reduced pressure.
   • Adjust safety relief valves accordingly, ensuring they comply with the new safe pressure limit.
   • Reassess starting capability of main engines with the adjusted pressure.
3. Notification and Documentation:
   • Inform the Bridge Team and Port Authorities, especially if manoeuvrability could be compromised.
   • Notify the Classification Society in writing, providing full details of the inspection findings, remedial actions, and recalculated pressure.
4. Prohibition of Unauthorised Repairs:
   • No repair, welding, patching, or machining is to be undertaken by ship crew.
   • All repairs on pressure vessels must be approved by the Flag Administration and carried out by certified personnel.